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 ```RG-8, RG-213, solid dielectric... 31 pF/ft.. If it's in a tube that just snugly fits where the braid used to be (or, you left the braid on..) then this number works.. if it's in a bigger tube, the capacitance is less. ----- Original Message ----- From: "Jim White, K4OJ" To: "Jim Lux" Cc: Sent: Saturday, June 21, 2003 3:11 PM Subject: Re: [TowerTalk] homemade beams > Now know why I wished there was graph/chart someplace with atypical values! > > Kinda like the great chart K5KG does with lengths of dipole/loops/etc... > > An "easy reference" for gamma capacitors... I would use the aluminum > tubing that was just big enough to accommodate dielectric...dunno what > size that translates to.... and then.... what spacing for each HF band... > > Seems like someone could build some formulae into an excel spreadsheet > for us QCAO types to find a reasonable guesstimate/starting point. > > Thanks, > > Jim, K4OJ > > > > Jim Lux wrote: > > There are two approaches.. > > If the tube you're sticking it into has an ID very close to that of the OD > > of the insulation of the inner coax, then you can probably use the databook > > values for the coax. > > > > If not, then you're looking at a bit more complex thing.. If you've got some > > magic way of holding the coax centered in the tube, then it can be > > considered as two coaxial capacitors in series, one with air dielectric, the > > other with whatever the coax used. If the coax isn't centered, it's a whole > > lot more complex. > > > > In any case, the formula for capacitance of coax is: > > > > C = 2 * Pi * Eo * Er * ln(router/rinner) * length > > > > Eo is the permittivity of free space 8.85 pF/m, Er is the relative > > dielectric constant: 1 for air, around 2-3 for most plastics > > router is the inside diameter of the outer conductor > > rinner is the OD of the inner conductor > > all dimensions in meters > > > > That should get you close... > > > > ----- Original Message ----- > > From: "Jim White, K4OJ" > > To: "Mirko Sibilja" > > Cc: "Gregg Seidl" ; ; "Floyd > > Sense" > > Sent: Saturday, June 21, 2003 2:00 PM > > Subject: Re: [TowerTalk] homemade beams > > > > > > > >>Do you know a location on the web where approximate lengths are > >>specified? I have used commercials antennas which this approach but > >>whenever it was time to design my own matching/gamma I took the easy > >>route and ordered a compete unit from Cushcraft and used theirs... > >> > >>I have not seen this as a sub routine on any of the antenna designing > >>software but mebbe I missed it... > >> > >>73, > >> > >>Jim, K4OJ > >> > >> > >> > >> > >>Mirko Sibilja wrote: > >> > >>>Another way to make the gamma match capacitor is to use piece of coax > >> > > cable > > > >>>with braid removed, inserted into gamma tube. Calculate the neccessary > >>>lenght by cable's capacity/length unit. > >>> > >>>S57AD > >>>----- Original Message ----- > >>>From: Floyd Sense > >>>To: Gregg Seidl ; > >>>Sent: Friday, June 20, 2003 7:15 PM > >>>Subject: Re: [TowerTalk] homemade beams > >>> > >>> > >>> > >>> > >>>>The gamma match cap is pretty easy. I've always made the cap from two > >>>>loosely telescoping pieces of tubing, covering the inside one with > >>> > > shrink > > > >>>>wrap tubing. That tube then serves as the gamma match tube and you > >>> > > adjust > > > >>>>the cap by moving the inner tube in or out. > >>>> > >>>>K8AC > >>>>----- Original Message ----- > >>>>From: "Gregg Seidl" > >>>>To: > >>>>Sent: Friday, June 20, 2003 10:14 AM > >>>>Subject: [TowerTalk] homemade beams > >>>> > >>>> > >>>> > >>>> > >>>>>I would like to know if any one has built any of the beams in the 19th > >>>>>edtion Antenna book by the ARRL.I'm considering building either a 3 or > >>>> > > 4 > > > >>>>>element 20 beam.I'm thinkimg about useing a hairpin match mostly > >>>> > > because > > > >>>I > >>> > >>> > >>>>>don't know how a gamma match works and where to find the cap that is > >>>>>needed.I know you need to insulate the driven elemnt from the boom on a > >>>>>hairpin but that I think could be done with some pvc or such,kinda like > >>>>>Mosley does.As a matter of fact I think I have an old Mosley center > >>>> > >>>>element > >>>> > >>>> > >>>>>in the shed.Any help would be great.I know I could never come close to > >>>>>builing a JRC JST-245 but I think I could build a 4 element beam just > >>>> > > as > > > >>>>>good as M2 or whatever and maybe save a few bucks to buy another > >>>> > >>>>JST-245,ok > >>>> > >>>> > >>>>>maybe not!!! Gregg K9KL > >>>>> > >>>>> > >>>>>_______________________________________________ > >>>>> > >>>>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless > >>>> > >>>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with > >>> > >>>any > >>> > >>> > >>>>questions and ask for Sherman, W2FLA. > >>>> > >>>> > >>>>>_______________________________________________ > >>>>>TowerTalk mailing list > >>>>>TowerTalk@contesting.com > >>>>>http://lists.contesting.com/mailman/listinfo/towertalk > >>>>> > >>>> > >>>> > >>>>_______________________________________________ > >>>> > >>>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless > >>> > >>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with > >> > > any > > > >>>questions and ask for Sherman, W2FLA. > >>> > >>> > >>>>_______________________________________________ > >>>>TowerTalk mailing list > >>>>TowerTalk@contesting.com > >>>>http://lists.contesting.com/mailman/listinfo/towertalk > >>> > >>> > >>>_______________________________________________ > >>> > >>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless > >> > > Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any > > questions and ask for Sherman, W2FLA. > > > >>>_______________________________________________ > >>>TowerTalk mailing list > >>>TowerTalk@contesting.com > >>>http://lists.contesting.com/mailman/listinfo/towertalk > >>> > >> > >> > >> > >>_______________________________________________ > >> > >>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless > > > > Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any > > questions and ask for Sherman, W2FLA. > > > >>_______________________________________________ > >>TowerTalk mailing list > >>TowerTalk@contesting.com > >>http://lists.contesting.com/mailman/listinfo/towertalk > > > > > > _______________________________________________ > > > > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. > > > > _______________________________________________ > > TowerTalk mailing list > > TowerTalk@contesting.com > > http://lists.contesting.com/mailman/listinfo/towertalk > > > > > ```